Conventionally, when the number of rotations and a rotational direction of a motor are detected, an encoder that includes sensors for obtaining information about both of an A-phase and a B-phase, has been used. The encoder, shown in FIG. 4, for example, includes a rotatable magnetic disk 15 and fixed side magnetic sensors 16, and obtains a rectangular wave signal from changes in output voltages of the magnetic sensors 16 according to a rotation of the magnetic disk 15 by passing the output voltages through a comparator.
Further, as shown in FIG. 5, a magnetic micro encoder 17, which is a small rotary encoder, includes a magnetic disc 19, two magnetic sensors 22a and 22b, and a housing 24. The magnetic disc 19 is axially magnetized and is mounted on a rotation shaft 18 together with a first back yoke 20. The two magnetic sensors 22a and 22b are arranged opposite to a surface of the magnetic disc 19 with a gap in an axial direction of the magnetic disc 19, and are mounted on a sensor mounting portion 21b of a flexible print substrate 21 in such a manner that longitudinal directions of the two magnetic sensors 22a and 22b are approximately parallel to a band-shaped wiring portion 21a of the flexible print substrate 21. Also, a second back yoke 23, which forms a magnetic circuit together with the first back yoke 20 and the magnetic disc 19, is mounted on the back of the two magnetic sensors 22a and 22b with the flexible print substrate 21 intervened therebetween. The housing 24 accommodates the first back yoke 20, the magnetic disc 19, the two magnetic sensors 22a and 22b, and the second back yoke 23. In this regard, a cut-out portion 24a, which draws out the band-shaped wiring portion 21a of the flexible print substrate 21 there through, is formed on the housing 24 in such a manner that the flexible print substrate 21 fits into the housing 24. These features being illustrated in International Laid-open Publication WO03/016829.
In regard to conventional small rotary encoders, a method of arranging magnetic sensors in a radial direction of a magnetic disk in view of the relationship between a housing space of the small rotary encoder, is considered in the arrangement of electronic components, such as the magnetic sensors or the like for detecting both of an A-phase and a B-phase. However, it is difficult to arrange the magnetic disk and the magnetic sensors in a housing space having a micro-sized diameter in that there is a limitation in terms of reducing a diameter of the magnetic disk or an arrangement space for the magnetic sensors is required to be in a radial direction of the magnetic disk.
Also, with regard to the magnetic disc, although the diameter of the housing can be reduced by fixedly arranging the magnetic sensors as disclosed in Patent Reference 1, it is necessary to insert and fit a substrate into the housing so that the magnetic sensors may be arranged in desired positions with respect to the magnetic disk. In this regard, a method of fitting the housing into the substrate by using an adhesive after loosely inserting the substrate into an inner circumference of the housing and a method of tightly fitting the substrate into the inner circumference of the housing are considered. Actually, in a small rotary encoder having an outer diameter of about 20 mm, a dimensional tolerance of the circumference of the substrate is large, about ±0.1 mm, and when the substrate tightly fits into the housing, the substrate may not fit into the housing due to enlargement of a tightening value. Also, when the substrate loosely fits into the housing, the magnetic sensors and the magnetic disk may not be fixedly arranged in desired positions due to an increase in a clearance. When the substrate loosely fits into the housing, the adhesive or the like is necessarily used to fix the substrate in the housing. In this case, however, since the substrate must be supported in a desired position until the adhesive is hardened, a jig is necessary and an increase in complication of processing cannot be avoided.
Therefore, the present invention provides a small rotary encoder capable of resolving the above problems, and can easily arrange and fix a substrate in a desired position without changing dimensional tolerances of a housing and the substrate, without requiring a jig used to harden the adhesive during an adhesion process.